Field check of a poker vibrator's radius of action is a visual, not an instrumented, test: the operator watches where air bubbles stop breaking the surface around the inserted head, and treats that distance as the working influence radius [S2][S6].
Two reference rules dominate the trade: spacing between insertions equals roughly 6 to 10 times the head diameter, and the influence radius itself is commonly taken as about 10× head diameter, with a conservative band of 4× in some guidance [S1][S3]. For a 38 mm head this works out to an influence radius near 380 mm and an insertion spacing near 350 mm [S3].
What the "radius of action" actually is
Radius of action is defined as the distance from the centreline of the vibrator head to the outer edge of the zone in which complete consolidation of the fresh concrete occurs [S2]. Outside that ring, the mix is no longer fully fluidised and entrapped air is not guaranteed to escape, which is why overlap between adjacent insertions is mandatory rather than optional [S2].
Two parameters drive that radius directly: head diameter and vibration amplitude. Larger head diameter raises the volume of concrete fluidised per insertion, and higher amplitude extends how far that fluidisation reaches into the surrounding mix; both parameters scale roughly linearly with eccentric mass and offset inside the head [S1]. Frequency sits in the 10,000–13,000 vpm band for most electric pokers in the 25–60 mm range, with pneumatic 75 mm heads pushed toward 16,000 vpm to compensate for their lighter construction [S3].
Field procedure: the bubble-escape check
The standard on-site method is to insert the poker vertically, let it sink under its own weight, hold it for 5 to 15 seconds, and watch the surface: the radius at which bubbles stop appearing marks the practical influence radius for that mix, slump, and head size [S2][S6]. The S2 guidance from Oztec adds that withdrawal should be slow enough to stay behind the upward air migration, allowing roughly 15 seconds for every 2 ft of lift to avoid re-trapping air [S2].
For documentation, mark the observed radius on the slab with a chalk circle after the first insertion, then re-insert at 1.0 to 1.5× that radius so the next zone overlaps the previous one by 25–50 percent; this is the same overlap principle that ACI 309-style practice has codified for decades and that BISON repeats in its 2023 operator note [S2][S3][S6]. When slump is high or a superplasticiser is in use, expect the real radius to roughly double the tabulated value, which means wider spacing is acceptable but not skipping coverage [S2].
Reference table: head size vs radius vs spacing
The Oztec table remains the most-cited field reference and pairs head size with radius of action and compaction rate in cubic yards per hour, shown here in metric for spec parity [S2]:
Head 3/4" (19 mm): radius ~75 mm, rate 1–3 yd³/hr. Head 1" (25 mm): radius ~100 mm, rate 2–4. Head 1-1/2" (38 mm): radius ~150 mm, rate 5–8. Head 1-3/4" (44 mm): radius ~225 mm, rate 8–16. Head 2" (50 mm): radius ~275 mm, rate 12–20 [S2].
Cross-checked against the 10× diameter rule from S3, a 50 mm head should give roughly 250 mm radius, close to the 275 mm in the Oztec table, while a 25 mm head lands near 100 mm on both references; the 38 mm example in the Excalibur case study sits at 380 mm radius, higher than Oztec's 150 mm because the 10× rule is the generous end of the band and the 4× conservative rule would put it near 150 mm [S2][S3]. For a deeper look at how the vibration energy actually moves through the mix, see the related piece on vibration debubbling in concrete.
Head diameter selection and what it does to the radius
Choosing the head is upstream of the radius check, but it sets the band you are about to verify in the field. ENAR's selection guide restricts pokers to the 25–100 mm range and ties diameter to reinforcement spacing, with a safety margin for the oscillation sweep so the head does not strike formwork [S1]. Excalibur's 2025 project guidance narrows the practical window to 25–90 mm and matches 25–35 mm heads to thin walls and dense rebar, 38–45 mm to general pours, 50–60 mm to heavy sections, and 75 mm pneumatic units to mass pours where the lighter head offsets the size [S3].
Two selection rules worth memorising: keep recommended layer depth at or below the head length, and at or below 1.5× head length for eccentric-electric types, and pour in overlapping 300–500 mm layers when total depth exceeds that [S3]. This is what makes the radius-of-action number translate into a verifiable, layer-by-layer procedure rather than a one-shot assumption.
Common on-site failure modes
Four mistakes show up repeatedly when the field check is done carelessly. First, using a small head in a stiff, large-aggregate mix: 25 mm heads clog and the fluidised zone collapses, so the bubble check returns a falsely small radius [S3]. Second, forcing the head down rather than letting it sink: this locks the poker between rebars and the operator never sees the real influence zone [S2]. Third, withdrawing too fast near the top, which churns air back into the cover zone and reads as a healthy radius when the cover is actually aerated [S2]. Fourth, treating nominal radius as fixed: high-slump or superplasticised mixes can roughly double the listed value, so a 50 mm head thought to cover 250 mm may in fact be fluidising 500 mm, which changes the spacing math [S2].
For equipment-heavy projects, the radius check also feeds back into operator endurance: larger heads at 50–60 mm draw 5,600–7,000 N of centrifugal force and tip the balance toward pneumatic or engine-driven units so the operator is not fighting a 16 kg electric head on a long lift [S3]. When the radius comes back smaller than the head-diameter rule predicts, the cause is almost always mix-related (stiff slump, low workability) rather than equipment-related.
Documentation and what to record
Treat the on-site check as a quality record, not a one-off observation. For each pour, log the head diameter, the observed bubble-escape radius, the slump at the point of placement, any admixture in use, the insertion spacing actually applied (1.0–1.5× radius), the hold time per insertion (5–15 s), and the lift rate (about 15 s per 600 mm of withdrawal) [S2]. The slump and admixture entries are what let the next shift reproduce the radius; without them, the next crew will guess and likely under-consolidate.
For QA purposes, the same radius-of-action logic that governs concrete vibrator selection also drives how other on-site tools are checked, but the bubble-escape method is specific to immersion pokers and does not transfer to form vibrators or surface screeds. Two signals worth tracking on the next pour: whether the observed radius trends lower as the mix stiffens through the day (flag water addition at the truck), and whether the spacing actually applied drifts above 1.5× radius under shift pressure (flag supervision, not the equipment).
Detailed specification references: concrete vibrator, pressure transmitter, and flow meter.